Viscous brines have been used in oil fields for many purposes, including packer fluids, fluid loss control pills, fracturing fluids, as well as other applications. For many of these applications, the viscosifiers are generally water-soluble polymers. The chemical and thermal stability of these water soluble polymers can be an issue, especially for long term applications, such as packer fluids, which in some instances may function for 10 years or more.
Well fluids can be employed in various applications in which they may be subject to high temperatures. Packer fluids, for instance, are inserted into an annulus between the well tubing and well casing above a packer, in order to control pressure differentials in the wellbore. Often, the packer fluid is employed to provide good insulation properties to prevent heat loss from the production to the environment. Polymer based packer fluids are well known in the art.
Water soluble lignin and modified lignin polymers have been used by industry in a variety of applications for many years, as an inexpensive polymer. Modified lignin has been used also in the oilfield primarily as a low-cost dispersant for clay based drilling fluids, such as for example, chrome lignosulfonate, titanium or zirconium lignosulfonate. Modified Lignin-based materials have also been used as dispersants for clay based drilling fluids where chrome cannot be used, such as the lignin-based grafted synthetic polymer available from LignoTech USA, Inc., Houston, Tex., sold under the name BORRETHIN F. As such, they are known to provide temperature stability and favorably control a fluids rheological properties. Drilling fluids can also contain supplemental fluid loss control polymers, and modified lignin dispersants are known to stabilize a formulated drilling fluid's rheological and fluid loss control properties to 250° F. and higher. Lignins and lignosulfonates have also been used in oilfield cement formulations as retarders, such as the low cost calcium and sodium lignin products, MARABOND 21 and KELIG 32, both available from LignoTech USA, Houston, Tex., and as emulsifiers for oil in aqueous fluids.
Water soluble polymers, such as polysaccharides, can have limited thermal stability, especially in high-density brine under high application temperatures. A polymer in brine can be subjected to several types of degradation, including hydrolysis, oxidation and free radical degradation, etc. Erythorbate and ascorbate type chemicals, such as sodium erythorbate, have been used to prevent oxidative degradation of polymers in water based systems in order to protect and prolong polymer functionality.
While advances have been made in well fluids, further improvements would be a welcome addition in the field. In particular, it would be desirable to provide well fluids comprising polymers having improved chemical and thermal stability.